(1) Field of the Invention
The present invention relates to a local area network (hereinafter abbreviated to LAN) for a vehicle with a double loop constitution and for a multiplex transmission of various signals in an automobile.
(2) Description of the Related Art
Hitherto, to simplify wiring inside a vehicle, it was proposed that a plurality of nodes be arranged in the automobile, the nodes being interconnected with one another by multiplex signal transmission lines. Switching signals, diagnostic signals, and the like from various apparatuses equipped on the vehicle could then be transmitted as a multiplex signal. Also, in order to withstand noise, the use of an optical fiber transmission line was proposed. Star type, loop type, and the like LAN systems using an optical fiber are known. In these systems, the loop type LAN is advantageous because the total length of the optical fiber is short and the optical fibers are simply arranged.
In the loop type LAN, however, a problem arises in that if only one portion of the loop breaks down or causes abnormal operation, none of the nodes will be able to transmit signals. To resolve this problem, another system has been proposed wherein the loop is formed in a double loop constitution, i.e., main-loop and sub-loop using this system, if the main-loop cannot transmit the signals, a portion of the sub-loop is used to transmit the signals as a loopback system instead of the main-loop; and the reliability against failure is increased.
Nevertheless, in this case, as long as the LAN is in a normal operation state the other (sub-loop) of the double loops is not necessary. Also, the size of the LAN system must be increased to enable use of this countermeasure to an abnormal operation, and this is very uneconomical.
The inventors of the present invention have conceived an effective application of the sub-loop in the case of the double loop constitution.
The vehicle signals transmitted by the LAN include signals such as an audio signal which transmits a large amount of information at a certain time interval, and signals generated intermittently at a certain time interval such as signals detecting an abnormal state, e.g., lamp breakdown or the like, and turning the switches ON or OFF in the various apparatuses.
Accordingly, another problem arises in that when an attempt is made to transmit all of the abovementioned signals by using only one transmission line which is comparatively small in scale, such as an LAN for a vehicle, the audio signal, for example, is band limited when played back.
The related arts for this invention are disclosed in Japanese Unexamined Patent Publications (Kokai) Nos. 58-70655 and 58-70658.
The present invention was made in consideration of the above-mentioned problems.